3GPP has standardized the mobility procedures between Generic Access Network (GAN) and cellular cells. One such procedure is “CS Handover from GAN”. In this procedure, the User Equipment (UE) in GAN mode with an active voice call initiates handover procedure to the cellular network by providing lists of neighbor candidate cells. The UE may for example be a mobile terminal such as, but not limited to a cellular telephone, a personal data assistant (PDA), or a wirelessly enabled computer. There may be two lists of candidate cells; one list consisting of 2G cells and the other consisting of 3G cells. Each list is sorted in the descending order of measured signal strengths for the candidate cells included in the list. The procedure allows reporting of the 2G list or the 3G list, or both the 2G and 3G lists. The procedure requires reporting of mobile country code (MCC), mobile network code (MNC), location area code (LAC) and Cell ID information for every cell in the candidate list.
If the UE prefers that the handover is to be initiated to a 3G (i.e. UMTS Terrestrial Radio Network (UTRAN)) cell, the UE has to report 3G neighbour cells in a candidate cell list for handover. In general the UE does not know all the required information about neighbourhood cells except the cell to which the UE's cellular Access Stratum (AS) is camping on in a “detached” state. The detached state occurs when the UE camps on GAN (i.e. higher layers are mapped to GAN AS) and one of its cellular AS receives cell broadcast information from one of the cellular cells.
Neighbour cell information in UTRAN is provided to the UE by one or more of System Information Block (SIB) 11, SIB 12 and SIB 11bis. The neighbour cell information includes UTRA Absolute Radio Frequency Channel Numbers (UARFCN) (for inter-frequency cells) and primary scrambling codes (PSC) (for inter-frequency and intra-frequency cells) for the neighbour cells. Using UARFCN and PSC of the 3G neighbour cells, the UE can generate a 3G candidate cell list for handover. The process involves synchronizing with each neighbour cell, measuring the neighbour cell signal strength and decoding Master Information Block (MIB) and SIB block information such as from SIB 1 or SIB 3, or both. The MIB contains MCC and MNC information, LAC information is provided in SIB 1 and Cell ID information is provided in SIB 3. This process is repeated for one or more cells in the neighborhood. Once the information is available for several 3G cells, then the UE sorts the list of cells in descending order of their measured signal strength and sends the list of 3G candidate cells to the Generic Access Network Controller (GANC). This process takes time and may put considerable processing load on the UE.
Furthermore, the GAN Controller (GANC), according to 3GPP Release 8 or earlier, does not announce its handover capability (implicitly or explicitly) to the UE. It is possible that a GANC is not capable of handover to 3G cells. In such a case, if the UE reports only a 3G candidate list then handover will not proceed. This may delay the handover. Therefore, for 3GPP Release 8 or earlier it is advisable for the UE to report a 2G candidate list along with the 3G candidate list in order to increase the chances of a successful handover. Sending both a 2G candidate list and 3G candidate list puts additional burden on the processing of the UE. The processing overhead becomes acute because of the possibility of rapid drop in the GAN signal quality which can result in a call being dropped before the UE furnishes the required candidate cell lists.
If the UE prefers that the handover is initiated to a GERAN cell instead of a 3G cell and the UE's GERAN AS is in the detached state, then the UE can provide several GERAN cells in the candidate cell list. There is less processing required from the UE because generating a 2G candidate cell list is easier than generating a 3G candidate cell list, and there is no need to report a 3G candidate list when handing over to a GERAN cell.